Add autofocus support for input.html templates

[jamesbeith]

Description

This change adds support to use 'autofocus': True in the style options and have the autofocus attribute included on the input field when rendered.

[lovelydinosaur]

👍

[jamesbeith]

@tomchristie I'd considered adding similar support for autocomplete too but then there's also pattern, step and other input attributes that might be of some use. Had you considered using a more freeform dictionary approach for adding attributes to fields similar to Django's Widget.attrs?
https://docs.djangoproject.com/en/1.10/ref/forms/widgets/#django.forms.Widget.attrs

[lovelydinosaur]

It's not really something that's come in for consideration, no.
Happy to accept particular proposals, but it's not obvious how we'd best fit it in: Don't esp. want to nest things under style any further, but don't want to just blindly pull everything through to the template either.